Electroluminescent displays have selectively illuminable regions for displaying information. Such displays have the advantage over competing technologies that they can be large, flexible and are relatively inexpensive.
Although electroluminescent lamps were known in the 1950's, these had a short lifetime and it was not until the 1980's that a flexible electroluminescent device was developed. However, this was used as an LCD backlight and only recently have practical electroluminescent displays become available.
Electroluminescent displays generally comprise a layer of phosphor material, such as a doped zinc sulphide powder, between two electrodes. It is usual for at least one electrode to be composed of a transparent material, such as indium tin oxide (ITO), provided on a transparent substrate, such as a polyester or polyethylene terephthalate (PET) film. The display may be formed by depositing electrode layers and phosphor layers onto the substrate, for example by screen printing, in which case opaque electrodes may be formed from conductive, for example silver-loaded, inks. Examples of electroluminescent devices are described in WO 00/72638 and WO 99/55121.
An electroluminescent display of the general type described above is illuminated by applying an alternating voltage of an appropriate frequency between the electrodes of the lamp to excite the phosphor. Commonly, the phosphors used in electroluminescent displays require a voltage of a few hundred volts. Typically, such electroluminescent displays may have a capacitance in the range 100 pF to 1 μF.
Since only a small current is required, this comparatively high drive voltage can easily be produced from a low voltage DC supply by a circuit such as the well known “flyback converter”.
This comprises an inductor and an oscillating switch arranged in series. In parallel with the oscillating switch, a diode and a capacitor are arranged in series. The switch oscillates between an open state and a closed state. In the closed state, a current flows from the DC supply through the inductor and the switch. When the switch is opened, the current path is interrupted, but the magnetic field associated with the inductor forces the current to keep flowing. The inductor therefore forces the current to flow through the diode to charge the capacitor. The diode prevents the capacitor discharging while the switch is closed. The capacitor can therefore be charged to a voltage that is higher than the DC supply voltage, and current at this voltage can be drawn from the capacitor.
In order to supply an alternating current to a load from a flyback converter, an H-bridge may be provided in parallel with the capacitor. In general, an H-bridge comprises two parallel limbs, each limb having a first switch in series with a second switch. On each limb between the first and second switches, there is a node, and the load is connected between the respective nodes of the limbs. Current can flow through the load in one direction via the first switch of one limb and the second switch of the other limb and in the other direction via the other two switches. The switches of the H-bridge are operated so that current flows through the load first in one direction and then in the other.
Where multiple electro-luminescent segments are provided to form a display, the segments are controlled by having a single high voltage rail of constant voltage that is selectively switched across the segments that are required to light. This is achieved by using a half H-bridge transistor configuration to drive a common, usually front, electrode and a number of half H-bridges to drive each of the multiple segments. The common electrode will be switched at a frequency in the region of a few tens of hertz to a few kilohertz. Segments that are not required to light will be driven with the same signal as the common electrode such that they see no net voltage. Segments required to light will be driven at the same frequency but in anti-phase with the common electrode such that they see an alternating voltage of peak-to-peak value that is twice that of the high voltage rail. This enables simple control of which segments light by control of the phase of their driving signals.
The brightness of all of the segments can be controlled by varying the voltage of the high voltage rail and/or by varying the switching frequency. The brightness of the segments increases with frequency. The constant voltage on the high voltage rail is usually achieved by means of a feedback loop that monitors the voltage and adjusts input power to eliminate variations. This arrangement is effective, but it does have some significant drawbacks. In particular, in some circumstances the display may be required to change rapidly leading to a sudden change in the power required to supply it. For example, the display may be required to change from having most segments lit to having few or none lit. This may lead to problems in regulating the voltage accurately. In addition, because it relies on maintaining a constant voltage on the supply rail regardless of load, the known arrangement tends to be rather inefficient in its use of power and this may be critical in the case of battery-operated devices.